Monday, March 2, 2009

Is there "life" below 350nm?

A lot has been discussed a forums dealing with UV photography what the low cut off UV wavelength might be of modern DSLR cameras i.e. how deep into the UV one could reach. A common assumption was around 350nm.

Well for that purpose I found a special steep astronomy filter which transmits only the 300-350nm band. A modified Nikon D70 camera was used, which I had modified using a clear quartz glass filter built in, which itself transmits flat from 270nm onwards. A Nikon UV-Nikkor f4.5 105mm was used as taking lens, since that transmits flat from 200nm onwards.

[click on image to see a larger version]

This now is the transmission spectra of that BP350 filter taken using a Ocean Optics USB2000 spectrometer (185-850nm; ignore that ripple on the right, measurement inaccuracy only, caused by the used Deutrium lamp):

and here in detail the spectra if stacked with a Baader U-filter (310-390nm) to insure NIR >700nm is completely suppressed, since that BP350 filter starts to leak some NIR above 900nm. The resulting transmission peaks at about 333nm and stretches from 320-350nm.

Now to the really interesting part, will the mod. D70 be able to record some useful information below 350nm? The camera was set to ISO1600 and the working aperture was f8. As a UV light source I used one shot of my modified High Power UV flash set at 400Ws, with UV front filter. Image came directly from the camera, only resized, and obviously only the RED channel records some useful information in this waveband.

So yes, there is life below 350nm, actually to about 325...330nm I would say.

And just for comparison, this is the result if only the Baader U-filter was used at the same f-stop:

It is obvious that using the BP350 and the 320-350nm band causes some 2...3 stops less sensitivity, but shows different detail of that Phalaenopsis Hybride Orchid.

Stay tuned, more will follow on that fascinating subject...

More info on this very interesting field may be found on my site